Permanent magnet member for coil motor and voice coil motor

ABSTRACT

A magnet body  1  including a shorter periphery  11 , a longer periphery  12  located at a position separated from the shorter periphery  11  by a predetermined distance, and a pair of side peripheries  13, 14  connecting the shorter periphery  11  and longer periphery  12  to each other, the magnet body having a fan-shaped planar form; and a corrosion-resistant film (Ni plating film  2 ) applied to a surface of the magnet body. The permanent magnet member  10  has a thickness whose maximum and minimum values yield a difference of 10 to 150 μm therebetween.

TECHNICAL FIELD

The present invention relates to a voice coil motor and a permanentmagnet member for the voice coil motor.

BACKGROUND ART

Hard disk drives (hereinafter referred to as “HDD”) in widespread usehave a structure in which one or a plurality of magnetic disks arrangedconcentrically are driven by a spindle motor. Reading and writing ofdata in the HDD is carried out by a magnetic head disposed so as tooppose the magnetic disk. The magnetic head is driven by an actuator. Asthe actuator, a swing operation type voice coil motor (hereinafterreferred to as “VCM”) is used in general.

With reference to FIG. 7, a typical configuration and operation of theVCM will now be explained. As depicted, the VCM comprises a pair ofyokes 15 disposed so as to oppose each other vertically, a permanentmagnet member 10 disposed between the pair of yokes 15 and bonded to thelower yoke 15; and a head carriage 17, disposed so as to be rotatableabout a shaft 18, having a fan-shaped coil 16 arranged in a magnetic gapformed between the upper yoke 15 and permanent magnet member 10.

When a predetermined current flows through the coil 16 in this VCM, adriving force occurs in any of directions of arrows A in the coil 16 inconformity to Fleming's left hand rule, whereby the head carriage 17rotates about the shaft 18 in any of directions of arrows B. Because ofsuch an action of the VCM, a magnetic head 19 mounted to the leading endpart of the head carriage 17 moves in any of directions of arrows Cwhich is opposite from the driving force generated in the coil 16. As aconsequence, the magnetic head 19 can be positioned with respect to amagnetic disk 20.

Employed as the permanent magnet member 10 used in the VCM is an R-T-Btype rare-earth permanent magnet material (wherein R is at least onekind of rare-earth element including Y, whereas T is at least one kindof transition metal element including Fe or Fe and Co as an essentialingredient), since it yields excellent magnetic characteristics. Thisrare-earth permanent magnetic material exhibits a low resistance tocorrosion, since R and Fe, which are main constitutional elementsthereof, are quite easy to oxidize. Therefore, when using this materialas the permanent magnet member 10, the surface of a magnet bodyconsisting of the permanent magnet material is usually coated with acorrosion-resistant film. For such a corrosion-resistant film, Ni or Nialloy plating, which is excellent in resistance to corrosion,reliability, cleanliness, etc., is often employed. As the yokes 15, onthe other hand, a silicon steel plate whose surface is provided withelectroless Ni plating is often used.

Meanwhile, for responding to the recent speedup in informationprocessing, HDDs employed as data storage means are required to bedriven at a higher speed. This makes it necessary for the magnetic disk20 to rotate at a high speed, which requires the VCM to be driven fastcorrespondingly thereto. In the conventional VCM, the permanent magnetmember is used in the state secured to the yoke 15 as mentioned abovetypically by way of an adhesive layer. In order for the VCM to fullysecure the durability at the time of high-speed driving, it is desirablethat the permanent magnet member 10 and the yoke 15 be bonded firmly toeach other.

Japanese Patent Application Laid-Open No. 2002-158105 discloses a methodin which the surface of the Ni plating film in the permanent magnetmember is phosphated with a processing solution having a specificcomposition. In this method, a phosphate coating having a desirablethickness is formed on the Ni plating film. The resulting magnet caneffectively eliminate the poor hardening of an adhesive which is notreactive on the Ni plating film. This can reduce fluctuations in thebonding strength due to the adhesive, and can attain a bonding strengthgreater than that conventionally available. As a result, a higherefficiency can be achieved in the bonding operation.

DISCLOSURE OF THE INVENTION

Thus, the permanent magnet member conventionally made by phosphating theNi plating film surface with a processing solution having a specificcomposition as mentioned above can improve the bonding strength whenbonding the magnet with the adhesive. However, as the permanent magnetmember, one which can exhibit excellent adhesion to yokes and the likewhen used in the VCM in particular has recently been in demand.

Therefore, it is an object of the present invention to provide apermanent magnet member for a VCM, which improves the adhesion to yokesby a technique different from the conventional technique mentionedabove. It is another object of the present invention to provide a VCMequipped with such a permanent magnet member.

For achieving the above-mentioned objects, the inventors studied therelationship between the form of a bonding surface of a permanent magnetmember for a VCM to a yoke and the bonding strength between thepermanent magnet member and yoke. As a result, the inventors have foundthat excellent adhesion to the yoke can be obtained when the thicknessof the permanent magnet member has maximum and minimum values yielding adifference therebetween falling within a predetermined range. This seemsto be because a space useful for holding an adhesive for bonding thepermanent magnet member for a VCM and the yoke together is formed at thebonding surface of the permanent magnet member to the yoke.

The present invention is based on the findings mentioned above andprovides a permanent magnet member for a VCM, the permanent magnetmember comprising a magnet body including a shorter periphery, a longerperiphery located at a position separated from the shorter periphery bya predetermined distance, and a pair of side peripheries connecting theshorter and longer peripheries to each other, the magnet body having afan-shaped planar form; and a corrosion-resistant film applied to asurface of the magnet body; wherein the permanent magnet member has athickness whose maximum and minimum values yield a difference of 10 to150 μm therebetween.

Thus, the permanent magnet member for a VCM has a thickness with maximumand minimum values. Therefore, the permanent magnet member for a VCMattains a space which can hold an adhesive on its bonding surface at thetime when bonded to a yoke in the VCM by way of an adhesive layer.Consequently, when the permanent magnet member is bonded to the yoke, agreater amount of adhesive is interposed therebetween than in theconventional case where flat surfaces are bonded together. As a result,the permanent magnet member is firmly bonded to the yoke.

Preferably, the maximum and minimum values of the thickness exist in thefollowing fashion in the permanent magnet member for a VCM. Namely, itis preferred that the maximum value of the thickness exist along aperipheral part comprising the shorter periphery, longer periphery, andside peripheries, and that the minimum value of the thickness exist inan area surrounded by the peripheral part.

In thus configured permanent magnet member for a VCM, a region extendingalong the peripheral part attains a form projecting from the areasurrounded by the peripheral part. The permanent magnet member for a VCMhaving such a form can hold the adhesive in the space formed in the areasurrounded by the peripheral part when bonded to the yoke. Therefore, agreater amount of adhesive can be held between the permanent magnetmember and the yoke, whereby the bonding strength therebetween furtherimproves.

The permanent magnet member for a VCM may have shapes formed by thefollowing modes. First, it is preferred that the corrosion-resistantfilm have a thickness whose maximum value exists along a peripheral partconstituted by the shorter periphery, longer periphery, and sideperipheries, and whose minimum value exists in an area surrounded by theperipheral part. In other words, this mode can be considered a statewhere the corrosion-resistant film is thicker in the peripheral partthan in other areas.

In this case, the magnet body may have a substantially uniform thicknessor a thickness smaller in the peripheral part than in other areas. Evenwhen the magnet body has such a form, the corrosion-resistant film isshaped as mentioned above, whereby the permanent magnet member for a VCMhas such a form that the peripheral part thereof projects.

On the other hand, the magnet body may have a thickness greater in theperipheral part thereof than in other areas. Preferably, in this case,the corrosion-resistant film has a substantially uniform thickness. Sucha permanent magnet member for a VCM also attains a form in which theperipheral part projects.

Preferably, the permanent magnet member for a VCM in accordance with thepresent invention has a thickness of 5 mm or less, whereas thecorrosion-resistant film is constituted by an electric plating film madeof Ni or an Ni alloy and has a thickness falling within the range of 5to 60 μm. Thus configured permanent magnet member for a VCM canfavorably be employed in typical VCMs and achieves high versatility.

In another aspect, the present invention provides a favorable VCMequipped with the permanent magnet member for a VCM in accordance withthe present invention. Namely, the VCM in accordance with the presentinvention comprises a pair of yokes disposed so as to oppose each otherwith a predetermined distance therebetween, a permanent magnet memberdisposed between the pair of yokes and bonded to each of the yokes byway of an adhesive layer, and a coil mounted to a rotatable member anddisposed in a magnetic space formed by the permanent magnet member andthe yoke, the rotatable member being rotatable about a predeterminedaxis; wherein a bonding surface of the permanent magnet member to theyoke has a peripheral part projecting by 5 to 75 μm from an areasurrounded by the peripheral part.

In thus configured VCM, the bonding surface of the permanent magnetmember to a yoke has the form mentioned above, whereby a space is formedbetween the permanent magnet member and the yoke. The adhesive layerbonding the permanent magnet member and the yoke to each other is formedso as to fill this space. Therefore, a greater amount of adhesive isheld between the permanent magnet member and yoke in thus configured VCMthan in the conventional case where the permanent magnet member and yokeare bonded by flat surfaces. As a result, the VCM becomes one in whichthe permanent magnet member and yoke are firmly bonded together.

As mentioned above, the permanent magnet member for a VCM in accordancewith the present invention has a predetermined space which can hold anadhesive on a surface to be bonded to a yoke. From such a viewpoint, thepermanent magnet member for a VCM in accordance with the presentinvention may be a planar permanent magnet member, the permanent magnetmember comprising a magnet body and a corrosion-resistant film formed ona surface of the magnet body, the permanent magnet member having firstand second surfaces opposing each other, at least one of the first andsecond surfaces being formed with a recess, a deepest part in the recessand a tangent plane thereof have a distance of 5 to 75 μm therebetween.

At the part to be bonded to a yoke, the permanent magnet member for aVCM having such a shape can form a space similar to that in the caseemploying the above-mentioned permanent magnet member for a VCM.

Preferably, thus specified permanent magnet member for a VCM has a shapesimilar to that mentioned above. Namely, it is preferred that thepermanent magnet member for a voice coil motor comprise a shorterperiphery, a longer periphery located at a position separated from theshorter periphery by a predetermined distance, and a pair of sideperipheries connecting the shorter and longer peripheries to each other,and have a fan-shaped planar form. More preferably, the first and secondsurfaces have a distance of 5 mm or less therebetween, and thecorrosion-resistant film has a thickness of 5 to 60 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing the permanent magnet member for a VCM inaccordance with an embodiment;

FIG. 2 is a view showing a first mode of the cross-sectional structureof the permanent magnet member taken along the line A-A of FIG. 1;

FIG. 3 is a view showing a second mode of the cross-sectional structureof the permanent magnet member taken along the line A-A of FIG. 1;

FIG. 4 is a view showing a third mode of the cross-sectional structureof the permanent magnet member taken along the line A-A of FIG. 1;

FIG. 5 is a perspective view showing the VCM in accordance with anembodiment;

FIG. 6 is a view schematically showing the cross-sectional structure atthe bonding part between the permanent magnet member for a VCM and ayoke in the VCM shown in FIG. 5;

FIG. 7 is a perspective view showing a conventional VCM; and

FIG. 8 is a view showing the cross-sectional structure of an embodimentof the permanent magnet member for a VCM taken along the line B-B ofFIG. 1.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, preferred embodiments of the present invention will beexplained in detail with reference to the drawings. Here, constituentsidentical to each other will be referred to with numerals identical toeach other without repeating their overlapping explanations.

The permanent magnet member for a VCM in accordance with an embodimentcomprises a magnet body and an Ni or Ni alloy plating (which willcollectively be referred to as “Ni plating”) acting as acorrosion-resistant film provided so as to cover the surface of themagnet body.

FIG. 1 is a plan view showing the permanent magnet member for a VCM inaccordance with this embodiment. FIGS. 2 to 4 are views showing first tothird modes of the cross-sectional structure of the permanent magnetmember for a VCM taken along the line A-A of FIG. 1. FIG. 8 is a viewshowing an embodiment of the cross-sectional structure of the permanentmagnet member for a VCM taken alone the line B-B of FIG. 1. Thepermanent magnet member 10 for a VCM comprises a shorter periphery 11, alonger periphery 12 opposing the shorter periphery 11 with apredetermined distance therebetween, and side peripheries 13 and 14connecting the shorter periphery 11 and longer periphery 12 to eachother. The permanent magnet member 10 has a planar form with an upperface 32 (first surface) and a bottom face 33 (second surface) which arelocated on the upper and lower sides of a peripheral part 30 constitutedby the shorter periphery 11, longer periphery 12, and side peripheries13, 14. This permanent magnet member 10 has a fan-shaped planar form.

Preferably, thus configured permanent magnet member 10 has a thicknessof 5 mm or less. Here, the thickness of the permanent magnet member 10refers to the distance between the upper face 32 and bottom face 33, andis represented by t in FIGS. 2 to 4 and 8. As depicted, the thickness tis the sum of the thickness t1 of the magnet body 1 and the thickness t2of the Ni coating film 2 acting as the corrosion-resistant film. Thethickness of the permanent magnet member 10 may be 3 or 2 mm or less asappropriate.

The permanent magnet member 10 is a thin permanent magnet member havinga flatness of 100 or greater, and has a planar form as mentioned above.Here, the flatness is defined by a value obtained when the area of theplanar part (area taken as a plane) of the permanent magnet member 10 isdivided by the thickness t of the permanent magnet member 10.

Here, the VCM in accordance with an embodiment will be explained withreference to FIGS. 5 and 6. FIG. 5 is a perspective view showing the VCMin accordance with this embodiment. FIG. 6 is a view schematicallyshowing the cross-sectional structure at the bonding part between thepermanent magnet member 10 and a yoke 115.

This VCM 100 comprises a pair of yokes 115, a pair of permanent magnetmembers 10 disposed between the pair of yokes 115 and bonded to therespective yokes 115, and a head carriage 117 (rotatable member)disposed so as to be rotatable about a shaft 118 and mounted with a coil116 positioned between the pair of permanent magnet members 10. The coil116 in the head carriage 117 is disposed within a magnetic space formedby the yokes 115 and the permanent magnet members 10. Each permanentmagnet member 10 is in contact with its corresponding yoke 115 by abonding surface 124.

In the VCM 100, each permanent magnet member 10 is bonded to itscorresponding yoke 115 by way of an adhesive layer 122 (see FIG. 6). Amagnetic head 119 for recording/reading data onto/from a disk 120 isdisposed at an end part of the head carriage 117 opposite from the coil116.

The VCM 100 operates as with the one explained above with reference toFIG. 7. Namely, when a predetermined current flows through the coil 116,a driving force occurs in any of directions of arrows A in the coil 116,whereby the head carriage 117 rotates about the shaft 118 in any ofdirections of arrows B. Because of such an action of the VCM, themagnetic head 119 mounted to the leading end part of the head carriage117 moves in any of directions of arrows C. As a consequence, themagnetic head 119 can be positioned with respect to a magnetic disk 120.

Referring to FIGS. 1 to 4 again, the permanent magnet member 10 will nowbe explained. The thickness t of the permanent magnet member 10 is notuniform but has maximum and minimum values. As mentioned above, thepermanent magnet member 10 is bonded and secured to the yoke 115 by wayof the adhesive layer 122 in the VCM 100.

The inventors conducted detailed studies and have found that thepermanent magnet member 10 can attain a high bonding strength to theyoke 115 when the difference (tmax−tmin) in thickness t between themaximum value (tmax) and the minimum value (tmin) falls within the rangeof 10 to 150 μm. This seems to be based on the following reason.

Since the permanent magnet member 10 for a VCM has such maximum andminimum values of the thickness t, at least one of the upper face 32 andbottom face 33 is formed with a recess 36 depressed toward the otherface. Hence, a space based on the recess 36 is formed at the bondingpart between the permanent magnet member 10 and the yoke 115. Since theyare bonded by way of the adhesive layer 122, the adhesive constitutingthe adhesive layer 122 is held within this space. The high bondingstrength mentioned above seems to be exhibited as a result of theadhesive held in such a space.

When the value of tmax−tmin is less than 10 μm, the bonding strength isless likely to improve sufficiently. When tmax−tmin exceeds 150 μm, bycontrast, the amount of adhesive held in the space increases so muchthat it takes a longer time to dry and solidify the adhesive, wherebythe bonding strength tends to become insufficient. Therefore, the valueof tmax−tmin is set to 10 to 150 μm in the permanent magnet member 10.This value is preferably 30 to 100 μm, more preferably 40 to 70 μm.

More preferably, the maximum and minimum values of the thickness t inthe permanent magnet member 10 are set such that the peripheral part 30of the permanent magnet member 10 is thicker than other parts. When thethickness of the peripheral part 30 is made greater as such, each of theupper face 32 and bottom face 33 of the permanent magnet member 10 has acrater-like recess 36, which can effectively hold the adhesive.

The difference between the maximum and minimum values is 10 to 150 μm,since it is set with reference to the thickness of the permanent magnetbody 10. However, the difference is not always required to be based onsuch a reference from the viewpoint of effectively forming the recess 36mentioned above.

When the peripheral part 30 projects from other areas as mentionedabove, for example, the adhesive can favorably be held in the spaceformed in an area 126 surrounded by the peripheral part 30. In thiscase, it will be sufficient if the amount of projection of the tmax partfrom the tmin part (i.e., the amount of projection of the peripheralpart 30 from other areas) is ½ of the tmax−tmin value. Namely, it willbe sufficient if the peripheral part 30 in the bonding surface 124 ofthe permanent magnet member 10 to the yoke 115 projects by 5 to 75 μmfrom other areas. Employable as a value indicating such an amount ofprojection is the distance t3 between the deepest part and tangent planeP of the recess 36 formed in the upper face 32 or bottom face 33 becauseof the projection of the peripheral part 30.

The following three techniques can be mentioned as those used in orderfor the tmax−tmin in the permanent magnet member 10 to attain a value of10 to 150 μm. The first technique makes the thickness t1 of the magnetbody 1 uniform, while changing the thickness t2 of the Ni plating film2. The second technique changes both the thickness t1 of the magnet body1 and the thickness t2 of the Ni plating film 2. The third techniquechanges the thickness t1 of the magnet body 1, while making thethickness t2 of the Ni plating film 2 uniform.

The above-mentioned techniques will now be explained. In each of thetechniques explained in the following, the upper face 32 or bottom face33 of the permanent magnet member 10 is formed with a crater-like recess36.

The first technique will now be explained. FIG. 2 is a view showing afirst mode of the cross-sectional structure of the permanent magnetmember taken along the line A-A of FIG. 1, and illustrates the permanentmagnet member 10 employing the first technique.

As shown in FIG. 2, the thickness t1 of the magnet body 1 is uniform inthe permanent magnet member 10 employing the first technique. On theother hand, the thickness t2 of the Ni plating film 2 is greater inregions extending along the shorter periphery 11 and longer periphery12. Therefore, the thickness t of the permanent magnet member 10 attainsthe maximum value in the regions extending along the shorter periphery11 and longer periphery 12. This thickness t attains the minimum valueat a center part c of the permanent magnet member 10, which is locatedbetween the shorter periphery 11 and longer periphery 12.

A method of forming the Ni plating film 2 in the first technique willnow be explained. The Ni plating film 2 can be formed by electroplating.At the time of electroplating, electric fields are applied to theperipheral part 30 in the permanent magnet member 10 from a plurality ofdirections. Therefore, the current density becomes higher in this part.On the other hand, electric fields are applied to the upper face 32 andbottom face 33 from vertical directions alone, whereby the currentdensity becomes lower there than in the above-mentioned part.

Utilizing such a difference in current density can make the thickness t2greater in the region extending along the shorter periphery 11 than inthe center part c as shown in FIG. 2. In the peripheral part 30, thecurrent density is higher in the regions circled with dash-single-dotlines in FIG. 1, i.e., the intersection between the shorter periphery 11and side periphery 13, the intersection between the shorter periphery 11and side periphery 14, the intersection between the longer periphery 12and side periphery 13, and the intersection between the longer periphery12 and side periphery 14, whereby the thickness t2 of the Ni platingfilm 2 is likely to become greater in these regions.

Among these regions, the current density is higher in the intersectionbetween the longer periphery 12 and side periphery 13 and theintersection between the longer periphery 12 and side periphery 14, eachyielding an acute angle of intersection, whereby the thickness t2 of theNi plating film 2 is likely to attain the maximum value there.

The second technique will now be explained. FIG. 3 is a view showing asecond mode of the cross-sectional structure of the permanent magnetmember taken along the line A-A of FIG. 1, and illustrates the permanentmagnet member 10 employing the second technique.

In the permanent magnet member 10 employing the second technique, asshown in FIG. 3, both the thickness t1 of the magnet body 1 and thethickness t2 of the Ni plating film 2 fluctuate. Namely, the thicknesst1 of the magnet body 1 is smaller in the region near the peripheralpart 30 (the shorter periphery 11 and longer periphery 12 in FIG. 3). Onthe other hand, the thickness t2 of the Ni plating film 2 is greater inregions extending along the shorter periphery 11 and longer periphery12.

In the regions extending along the shorter periphery 11 and longerperiphery 12, the thickness t2 of the Ni plating film 2 is set greaterthan the thickness by which the magnet body 1 is thinned in theperipheral part 30. Hence, the permanent magnet member 10 for a VCM isthicker in the peripheral part 30, whereby each of the upper face 32 andbottom face 33 is formed with a crater-like recess 36.

Thus, the thickness t attains the maximum value in the peripheral part30 and the minimum value in the center part c existing between theshorter periphery 11 and longer periphery 12 also in the permanentmagnet member 10 employing the second technique.

The shape of the magnet body 1 and Ni plating film 2 in the secondtechnique mentioned above can be formed in the following manner.Employable as methods of reducing the thickness t of the magnet body 1in the region near the peripheral part 30 include those subjecting themagnet body 1 to barrel polishing, etching with an acid before formingthe Ni plating film 2, and the like.

In each of these methods, conditions are set appropriately so as to thinthe region near the peripheral part 30. For etching, for example, thetime for etching may be adjusted, etc. When reducing the thickness t1 ofthe magnet body 1 in the region near the peripheral part 30 as such, itwill be preferred if this region is made thinner than other regions byabout 20 to 100 μm. The Ni plating film 2 can be formed as in the firsttechnique.

The third technique will now be explained. FIG. 4 is a view showing athird mode of the cross-sectional structure of the permanent magnetmember taken along the line A-A of FIG. 1, and illustrates the permanentmagnet member 10 employing the third technique.

As shown in FIG. 4, the thickness t2 of the Ni plating film 2 is uniformin the permanent magnet member 10 employing the third technique. On theother hand, the magnet body 1 is thicker in regions extending along theshorter periphery 11 and longer periphery 12. Therefore, the thickness tattains the maximum value in the peripheral part 30 and the minimumvalue in the center part c located between the shorter periphery 11 andlonger periphery 12 in this permanent magnet member 10 as well.

For making the thickness t2 uniform in the Ni plating film 2 as such, itwill be sufficient if the current density is held low at the time ofelectroplating. The magnet body 1 having the shape mentioned above canbe formed by barrel polishing or etching under a condition where thethickness t1 becomes smaller in the region near the center part cthereof.

Examples of the permanent magnet member for a VCM in accordance withpreferred embodiments include those in the modes mentioned above. In thepermanent magnet member 10 in accordance with any of the embodiments, itwill be preferred if the thickness t2 of the Ni plating film 2 is 5 to60 μm.

If the thickness of the Ni plating film 2 is less than 5 μm, pinholesare likely to be formed in the Ni plating film 2 even when the magnetbody 1 is mirror-polished. When such a pinhole is formed in the Niplating film 2, the corrosion of the magnet body 1 may proceed from thispart. When t2 is large, on the other hand, the volume of the magnet bodyin the permanent magnet member 10 becomes relatively smaller, wherebymagnetic characteristics of the permanent magnet member 10 tend todeteriorate. For avoiding such inconveniences, t2 is preferably 60 μm orless.

From the viewpoint of attaining sufficient corrosion-resistant andmagnetic characteristics, the thickness t2 of the Ni plating film 2 ismore preferably 10 to 30 μm. As mentioned above, the thickness t2 is notconstant but often fluctuates in the permanent magnet member 10. Evenwhen the thickness t2 fluctuates as such, it is preferred that t2 fallwithin the thickness range mentioned above in all the regions. For suchan Ni plating film 2, rack plating or barrel plating is selected asappropriate.

As the Ni plating film to be formed in the permanent magnet member 10,various kinds of Ni plating films can be applied without greatlychanging the adhesion. Hence, for improving the bonding strength betweenthe magnet body 1 and the Ni plating film 2, an undercoat layer (notdepicted) which can reduce the stress exerted on the interface betweenthe magnet body 1 and Ni plating film 2 may be provided therebetween.Preferably, the undercoat layer contains Cu as a main ingredient.

When an excess load is exerted on the bonded permanent magnet member 10for a VCM, a soft undercoat layer made of Cu, for example, can restrainthe stress generated between the magnet body 1 and the Ni plating film 2from concentrating at one location. This enhances the bonding strengthbetween the magnet body 1 and the Ni plating film 2. Though notrestricted in particular, the thickness of such an undercoat layer ispreferably 5 to 10 μm. In the permanent magnet member for a VCM in thepresent invention, such an undercoat layer is included in the concept ofthe corrosion-resistant film.

The magnet body 1 in the permanent magnet member 10 will now beexplained.

The magnet body 1 in accordance with an embodiment is an R-TM-B typerare-earth magnet containing R (where R is at least one kind ofrare-earth element including Y), TM (where TM is at least one kind oftransition metal including Fe or Fe and Co as an essential ingredient),and B.

Preferred as the rare-earth element R is one containing at least onekind selected from Nd, Pr, Ho, and Tb, or one containing at least onekind selected from La, Sm, Ce, Gd, Er, Eu, Pm, Tm, Yb, and Y in additionthereto. When two or more kinds of elements are contained as R, amixture such as misch metal can also be used as a material therefor.

Preferably, in the magnet body 1, the R content is 5.5 to 30 atom %. Ifthe R content is too small, i.e., less than 5.5 atom %, the crystalstructure of the magnet becomes a cubic system substantially identicalto that of α iron, thereby being harder to attain a high coercive force(the coercive force being referred to as “iHc” in the following). If theR content is too large, i.e., more than 30 atom %, R-rich nonmagneticphases increase too much, whereby the residual magnetic flux density(which will hereinafter be referred to as “Br”) tends to decrease.

The TM content is preferably 42 to 90 atom %. If the TM content is toosmall (less than 42 atom %), Br tends to decrease. If the TM content istoo much (more than 90 atom %), iHc tends to decrease. As TM, Co may becontained in addition to Fe. This can improve temperaturecharacteristics without lowering magnetic characteristics. Preferably,in this case, the amount of Co substituting Fe is 50% or less. If theamount of substitution by Co exceeds 50%, magnetic characteristics maydeteriorate.

The B content is preferably 2 to 28 atom %. If the B content is toosmall (less than 2 atom %), the crystal structure of the magnet becomesa rhombohedral structure, whereby iHc tends to become insufficient. Ifthe B content is too much (more than 28 atom %), B-rich nonmagneticphases increase too much, whereby Br tends to decrease.

The R-TM-B type rare-earth magnet constituting the magnet body 1 maycontain Ni, Si, Al, Cu, Ca, and the like as inevitable impurities by anamount of 3 atom % or less of the total in addition to theabove-mentioned R, TM, and B.

B may be partly substituted by at least one kind of element selectedfrom C, P, S, and Cu. This makes it easier to manufacture the magnetbody, thereby improving the productivity and lowering the manufacturingcost. Preferably, in this case, the amount of substitution is 4 atom %or less of the total. From the viewpoints of increasing iHc, improvingthe productivity, lowering the manufacturing cost, etc., at least onekind of element such as Al, Ti, V, Cr, Mn, Bi, Nb, Ta, Mo, W, Sb, Ge,Sn, Zr, Ni, Si, and Hf may be added. Their amount of addition ispreferably within a range not affecting magnetic characteristics, 10atom % or less of the total amount of constitutional atoms inparticular.

The magnet body 1 having the above-mentioned configuration comprises amain phase substantially in a cubic crystal structure. Preferably, themain phase has a particle size on the order of 1 to 100 μm. The magnetbody 1 contains 1 to 50% by volume of nonmagnetic phases.

A method of making the permanent magnet member 10 for a VCM inaccordance with a preferred embodiment will now be explained.

In the making of the permanent magnet member 10, the magnet body 1 ismanufactured at first. The magnet body 1 is favorably made by powdermetallurgy. The making of the magnet body 1 by powder metallurgy can becarried out in the following manner.

First, an alloy having a desirable composition is prepared by a knownalloy making process such as casting and strip casting. Subsequently,thus obtained alloy is roughly pulverized by a rough pulverizer such asjaw crusher, Brown mill, and stamp mill so as to attain a particle sizeof 10 to 100 μm, and then is finely pulverized by a fine pulverizer suchas jet mill and attritor so as to yield a particle size of 0.5 to 5 μm.

Thus obtained powder is molded preferably with a pressure appliedthereon in a magnetic field. Preferably, the magnetic field strength atthe time of molding is 955 to 1353 kA/m (12.0 to 17.0 kOe). The moldingpressure is preferably on the order of 0.5 to 5 tons/cm². Subsequently,thus molded product is sintered for 0.5 to 10 hours at a temperature of1000° C. to 1200° C. and then is cooled rapidly, so as to yield asintered body. Preferably, the atmosphere at the time of sintering is aninert gas such as Ar gas.

Thus sintered product is subjected to heat treatment (aging treatment)for 1 to 5 hours at a temperature of 500° C. to 900° C. preferably in aninert gas atmosphere, so as to yield the magnet body 1. The agingtreatment may be carried out in two stages as well. When the agingtreatment is carried out in two stages, it will be effective if thesintered product is held for predetermined periods of time in thevicinity of 800° C. and 600° C., respectively. When heat treatment iscarried out in the vicinity of 800° C. after the sintering, the coerciveforce of the magnet body 1 tends to increase in particular, which isspecifically effective in the mixing method. Since the coercive forcetends to be enhanced greatly by the heat treatment in the vicinity of600° C., it is preferred that the aging treatment be carried out in thevicinity of 600° C. in the case of a single stage. Thus obtained magnetbody 1 exhibits excellent magnetic characteristics in particular when Ris Nd. However, the magnet body 1 is known to yield a negativecoefficient of expansion in the direction perpendicular to the C axis.

After the magnet body 1 is formed as such, its surface is preferablysubjected to a predetermined process before forming the Ni plating film2. Specifically, after being degreased, the surface of the magnet body 1is chemically etched with an acid and is subjected to pretreatment forcleaning. The pretreatment is an arbitrary process, which is not alwaysnecessary. However, this pretreatment can eliminate dirt from thesurface of the magnet body 1, whereby the Ni plating film 2 can beformed favorably. The magnet body 1 may be subjected to barrel polishingfor removing burrs and the like from the surface before the degreasing.

Any degreasing solution usually employed for steel and the like can beused without any restrictions in particular. Employable as such adegreasing solution in general is one containing NaOH as a mainingredient and, if necessary, additives.

Preferably, nitric acid is used as an acid in the chemical etching. Whentypical steel materials are subjected to plating, nonoxidizing acidssuch as hydrochloric acid and sulfuric acid are often used in thechemical etching for pretreatment. When one containing a rare-earthelement, such as the magnet body 1, is treated with these nonoxidizingacids, however, hydrogen generated by the acids may be occluded in thesurface of the magnet body 1. This may weaken the site of occlusion,thereby generating a large amount of powdery undissolved matters. Suchpowdery undissolved matter remains on the surface of the magnet body 1even after surface treatment and leading this surface rough, therebycausing defects and poor adhesion in the Ni plating film 2 formed on thesurface. It is therefore desirable that the etching solution for themagnet body 1 containing a rare-earth element contain no nonoxidizingacids mentioned above.

Therefore, it is preferred that nitric acid, which is an oxidizing acidless likely to generate hydrogen, be used for chemically etching themagnet body 1. Preferably, the chemical etching solution containsaldonic acid or a salt thereof in addition to nitric acid. Aldonic acidor a salt thereof acts to form minute irregularities which cannot beseen with eyes on the surface of the magnet body 1. When the magnet body1 is formed with a number of such irregularities, the permanent magnetmember 10 attains similar irregularities on the surface thereof afterforming the Ni plating film 2.

The permanent magnet member 10 having such irregularities on the surfaceexhibits quite excellent adhesion with respect to the adhesive layer122. As a result, the bonding between the permanent magnet member 10 andthe yoke 115 is further improved. Such an action of forming minuteirregularities on the surface of the magnet body 1 is specific toaldonic acid and its salts and cannot be achieved by other organic acidssuch as citric acid and tartaric acid, for example.

The amount of dissolution of the surface of the magnet body 1 by thepretreatment is such that the magnet body is eliminated from the surfacepreferably by at least 5 μm, more preferably by 10 to 15 μm. If theamount of dissolution is too small, the altered layer or oxidized layerformed by processing the surface of the magnet body 1 may not beeliminated sufficiently, thus making it harder to form the Ni platingfilm 2 favorably thereon. This may remarkably deteriorate thecorrosion-resistant characteristic of the permanent magnet member 10.

The nitric acid concentration in the processing solution used for thepretreatment is preferably 1 normal or less, more preferably 0.5 normalor less. If the nitric acid concentration is too high (more than 1normal), the dissolving rate of the magnet body 1 may become too high,thereby making it harder to regulate the amount of dissolution. Whencarrying out mass-processing such as barrel processing in this case, theamount of dissolution fluctuates greatly among individual magnet bodies,whereby dimensional accuracy is harder to keep in the product. If thenitric acid concentration is too low, on the other hand, the amount ofdissolution tends to become insufficient. Therefore, the nitric acidconcentration is preferably 1 normal or less, more preferably 0.5 to0.05 normal. The amount of Fe dissolved in the processing solution atthe time when the treatment is completed is set to about 1 to 10 g/L.

Preferably, after the pretreatment, the magnet body 1 is further washedwith ultrasonic waves. Such ultrasonic washing can substantiallycompletely eliminate the small amount of undissolved matters andresidual acid components remaining in the magnet body 1. Preferably,such ultrasonic washing is carried out with ion-exchanged water whoseamount of chlorine ions, which may generate rust on the surface of themagnet body 1, is very small. If necessary, washing with similarion-exchanged water may be carried out before and after the ultrasonicwashing and before and after the pretreatment.

Thereafter, by electroplating, the Ni plating film 2 is formed on thesurface of the pretreated magnet body 1. Such electroplating can formthe Ni plating film 2, which is a high-performance corrosion-resistantfilm, at a low cost. Examples of the plating bath used for theelectroplating include Watt's bath containing no chlorinated Ni,sulfamic acid bath, borofluoric bath, and brominated Ni bath.

The permanent magnet member for a VCM and the VCM equipped therewith inaccordance with the present invention are not limited to theabove-mentioned embodiments, and can be modified in various mannerswithin the scope not deviating from the gist thereof. For example,though the VCM 100 in accordance with the above-mentioned embodiment isone in which the respective permanent magnet members 10 are bonded to apair of yokes 115, a single permanent magnet member 10 may be bonded toone of the yokes 115 alone as in the VCM shown in FIG. 7. From theviewpoint of effectively reducing the vibration generated from the VCM,it is preferred that the permanent magnet members 10 be bonded to bothof the yokes 115.

EXAMPLES

In the following, the present invention will be explained in furtherdetail with reference to examples, which do not restrict the presentinvention.

(Making of Permanent Magnet Member for VCM)

First, an alloy ingot having a composition constituted by 13.8 atom % ofNi, 1.2 atom % of Dy, 77.1 atom % of Fe, 1.1 atom % of Co, and 6.8 atom% of B was obtained. This ingot was subjected to hydrogen pulverization,in which hydrogen was occluded therein at room temperature and thendehydrogenation was effected for 1 hour at 600° C. in an Ar atmosphere.Subsequently, the hydrogen-pulverized alloy was roughly pulverized witha jaw crusher, and then finely pulverized with a jet mill, whereby finepowder having an average particle size of 3.5 μm was obtained.

Thus obtained fine particle was molded at a pressure of 1.2 tons/cm² ina magnetic field of 1194 kA/m (15 kOe), whereby a molded product wasobtained. Subsequently, the molded product was sintered for 2 hours at1100° C., so as to yield a sintered magnet. Then, the sintered magnetwas subjected to two stages of aging treatment, i.e., 1 hour at 800° C.and 2.5 hours at 550° C. (both in an Ar atmosphere).

Thereafter, the sintered magnet was cut into the form shown in FIG. 1,whereby the magnet body 1 was obtained. When cutting the magnet body 1,four kinds of magnet bodies with different thicknesses were cut outwhile the planar part area (area of the magnet body 1 taken as a plane)was held at 280 mm². Specifically, as shown in Table 1, theirthicknesses were 1.370 mm (No. 1), 1.460 mm (No. 2), 1.410 mm (No. 3),and 1.300 mm (No. 5). Each of these magnet bodies 1 exhibited a flatnessof 100 or greater.

In parallel with the above, the cut magnet body 1 was subjected tobarrel polishing and then etching, whereby a magnet body 1 (No. 4)having a magnet center part with a thickness of 1.470 mm and acute angleend parts with a thickness of 1.300 mm was obtained. These magnet bodies1 were subjected to barrel polishing, so that their peripheral partswere chamfered to R=0.5 mm, and then were immersed in an alkalinedegreasing solution. Thereafter, they were etched for 10 minutes with a3% aqueous nitric acid solution at 30° C.

Thus obtained samples Nos. 1 to 5 of magnet bodies 1 were subsequentlysubjected to barrel polishing, degreasing, and etching, and then waselectroplated with Ni by a barrel method using a Watt's bath, wherebythe Ni plating film 2 was formed on the surface of each magnet body 1.As a consequence, samples Nos. 1 to 5 of permanent magnet members 10 forVCMs (VCM magnets) corresponding to the samples Nos. 1 to 5 of magnetbodies 1 were obtained.

The thicknesses t2 in the center part (region indicated by c in FIGS. 2to 4 and 8) and acute angle end parts (the intersections between thelonger periphery 12 and side periphery 13 and between the longerperiphery 12 and side periphery 14) of the Ni plating films 2 formed onthe respective magnet bodies 1 were as shown in Table 1. The Ni platingfilms 2 for the respective permanent magnet members 10 were formed whilethe current density and plating time were set as shown in Table 2.

(Measurement of Thickness and Magnetic Flux of Permanent Magnet Memberfor VCM)

The thickness (thickness t1 of the magnet body+thickness t2 of Niplating film 2) was measured in the center part and acute angle endparts in each of thus obtained samples Nos. 1 to 5 of permanent magnetmembers 10, and the thickness ratio and the maximum value−minimum valueof thickness (max−min in Table 1) were calculated according to thesevalues. Also, the magnetic flux (μWbT) of each permanent magnet member10 was measured. Table 1 lists these results. Here, the thickness ratiorefers to the value calculated as the ratio of the thickness of themagnet body 1 in each of samples Nos. 1 to 4 of permanent magnet memberswith reference to the thickness of the magnet body 1 in sample No. 5 ofpermanent magnet member.

TABLE 1 Magnet body thickness (mm) Plating VCM magnet Cen- thickness(μm) Thickness Thickness in Magnetic Shear ter Acute Center Acute inCenter Acute angle Thickness max–min flux strength No part angle endpart angle end part (mm) end (mm) ratio (μm) (μWbT) (kgf/cm²) 1 1.370 1515 1.400 1.500 1.054 100 382 63 2 1.460 15 15 1.500 1.123 0 409 61 31.410 15 35 1.440 1.085 60 393 65 4 1.470 1.300 15 100 1.500 1.130 0 40460 5 1.300 15 100 1.330 1.000 170 362 45

TABLE 2 Average cathode current density Plating time No (A/dm²) (h) 10.03 50 2 0.03 50 3 0.1 1.7 4 0.3 5 5 0.3 5

In each of thus obtained permanent magnet members 10, the peripheralpart 30 projected from the area surrounded by the peripheral part 30.Also, as shown in Table 1, the thickness attained the maximum value(max) of 1.500 mm at the acute angle end parts, and the minimum value(mm) at the center part c (see FIGS. 2 to 4 and 8).

These permanent magnet members 10 are restricted in terms of their size,thickness in particular, since they are mounted to a VCM. Therefore,from the viewpoint of securing higher magnetic characteristics, it isimportant for each permanent magnet member 10 for a VCM to be designedas thick as possible within the range mountable to the VCM.

As shown in Table 1, each of samples Nos. 1 to 4 of permanent magnetmembers yielded a magnetic flux greater than that of sample No. 5 of thepermanent magnet member. This has proved that higher magneticcharacteristics are obtained as the thickness t1 of the magnet body 1 isgreater when the thickness t of the permanent magnet member 10 is thesame. However, the minimum value of thickness in the Ni plating film 2is preferably 5 μm or greater, since it becomes harder for the permanentmagnet member 10 to attain a sufficient corrosion-resistantcharacteristic if the Ni plating film 2 is too thin. Since the permanentmagnet member 10 having the Ni plating film 2 with a thickness of 15 μmexhibited an excellent corrosion-resistant characteristic as in thisexample, the thickness of the Ni plating film 2 is preferably at least15 μm.

(Evaluation of Bonding Strength)

Using samples Nos. 1 to 5 of permanent magnet members 10, bonding testswere carried out, so as to evaluate the bonding strength to a yoke 115which was a rotating member of a VCM. First, the permanent magnetmembers 10 were bonded to the respective yokes 115 with an anaerobicacrylic adhesive (Loctite 638UV manufactured by Loctite Japan Co.,Ltd.), so as to yield bonded products.

As the yoke 115, one comprising a body formed from a silicon steel plateand electroless Ni plating applied to the surface thereof was employed.The bonding was effected by coating a planar part (upper face 32 orbottom face 33) of the permanent magnet member 10 with 0.008 to 0.010 gof the adhesive, pressing this surface against the yoke 115, and holdingthe resulting bonded product for 30 minutes in a dryer whose temperaturehad been raised to 100° C. beforehand.

The bonded products comprising the respective permanent magnet memberswere subjected to a compression shear test at a rate of 5 mm/min at roomtemperature, so as to measure the shear strength (kgf/cm²) of eachbonded product. Table 1 lists thus obtained results.

From Table 1, the shear strength was seen to vary depending on themaximum value−minimum value (max−min in Table 1) in thickness of thepermanent magnet member 10. Specifically, the shear strength improved asthe maximum value−minimum value in thickness of the permanent magnetmember 10 increased from 0 μm (Nos. 2 and 4) to 60 μm (No. 3) and 100 μm(No. 1). This indicates that a greater bonding strength is obtained inthe case where the bonding surface has a space capable of holding theadhesive than in the case where completely flat surfaces are bondedtogether.

When the maximum value−minimum value was further increased to 170 μm(No. 5), however, the shear strength decreased on the contrary. Thisseems to be because the space for holding the adhesive is so large thatthe adhesive is applied to the magnet body 1 in excess, whereby theorganic solvent contained in the adhesive is harder to evaporate. It ispresumed that, as a result, the adhesive fails to solidify sufficiently,thereby lowering the bonding strength (shear strength).

INDUSTRIAL APPLICABILITY

Thus, the present invention can provide a permanent magnet member foruse in a voice coil motor (VCM), which improves the adhesion to yokes.The present invention can also provide a VCM equipped with such apermanent magnet member for a VCM, which can be driven at a higherspeed.

1. A permanent magnet member for a voice coil motor, the permanentmagnet member comprising: a magnet body including a shorter periphery, alonger periphery located at a position separated from the shorterperiphery by a predetermined distance, and a pair of side peripheriesconnecting the shorter and longer peripheries to each other, the magnetbody having a fan-shaped planar form; and a corrosion-resistant filmapplied to a surface of the magnet body; wherein the permanent magnetmember has a thickness whose maximum and minimum values yield adifference of 10 to 150 μm therebetween, and the corrosion-resistantfilm has a thickness whose maximum value exists along a peripheral partconstituted by the shorter periphery, longer periphery, and sideperipheries, and whose minimum value exists in an area surrounded by theperipheral part.
 2. A permanent magnet member for a voice coil motoraccording to claim 1, wherein the maximum value of the thickness existsalong a peripheral part comprising the shorter periphery, longerperiphery, and side peripheries; and wherein the minimum value of thethickness exists in an area surrounded by the peripheral part.
 3. Apermanent magnet member for a voice coil motor according to claim 2,wherein the corrosion-resistant film has a thickness greater in theperipheral part than in other areas.
 4. A permanent magnet member for avoice coil motor according to claim 3, wherein the magnet body has asubstantially uniform thickness or a thickness smaller in the peripheralpart than in other areas.
 5. A permanent magnet member for a voice coilmotor according to claim 2, wherein the magnet body has a thicknessgreater in the peripheral part than in the other areas; and wherein thecorrosion-resistant film has a substantially uniform thickness.
 6. Apermanent magnet member for a voice coil motor according to claim 1,wherein the permanent magnet member for the voice coil motor has athickness of 5 mm or less; wherein the corrosion-resistant film isconstituted by an electroplating film made of Ni or an Ni alloy; andwherein the corrosion-resistant film has a thickness falling within therange of 5 to 60 μm.
 7. A voice coil motor comprising: a pair of yokesdisposed so as to oppose each other with a predetermined distancetherebetween; a permanent magnet member according to claim 1 disposedbetween the pair of yokes and bonded to each of the yokes by way of anadhesive layer; and a coil mounted to a rotatable member and disposed ina magnetic space formed by the permanent magnet member and the yoke, therotatable member being rotatable about a predetermined axis; wherein abonding surface of the permanent magnet member to the yoke has aperipheral part projecting by 5 to 75 μm from an area surrounded by theperipheral part.